A ticket is a single customer request tracked in a helpdesk, with its own status, owner, tags and message history from first contact to close.
Tickets usually move through statuses such as open, pending (waiting on the customer) and closed. Each one can be assigned to a person, tagged by topic and measured for first response time and resolution time.
The word comes from IT service desks. Many ecommerce teams now say conversation instead, since a customer chatting about their order does not think of it as raising a ticket. In practice the two words describe the same unit of work.
Hobson calls them conversations and drafts a reply on each one. If you are moving from another helpdesk, past tickets, customers, macros and tags can come with you, see switching from Gorgias.